Lecture 9: A closer look at terms

Transcription

1 Lecture 9: A closer look at terms Theory Introduce the == predicate Take a closer look at term structure Introduce strings in Prolog Introduce operators Exercises Exercises of LPN: 9.1, 9.2, 9.3, 9.4, 9.5 Practical session

2 Comparing terms: ==/2 Prolog contains an important predicate for comparing terms This is the identity predicate ==/2 The identity predicate ==/2 does not instantiate variables, that is, it behaves differently from =/2

3 Comparing terms: ==/2 Prolog contains an important predicate for comparing terms This is the identity predicate ==/2 The identity predicate ==/2 does not instantiate variables, that is, it behaves differently from =/2?- a==a.?- a==b. no?- a=='a'.?- a==x. X = _443 no

4 Comparing variables Two different uninstantiated variables are not identical terms Variables instantiated with a term T are identical to T

5 Comparing variables Two different uninstantiated variables are not identical terms Variables instantiated with a term T are identical to T?- X==X. X = _443?- Y==X. Y = _442 X = _443 no?- a=u, a==u. U = _443

6 Comparing terms: \==/2 The predicate \==/2 is defined so that it succeeds in precisely those cases where ==/2 fails In other words, it succeeds whenever two terms are not identical, and fails otherwise

7 Comparing terms: \==/2 The predicate \==/2 is defined so that it succeeds in precisely those cases where ==/2 fails In other words, it succeeds whenever two terms are not identical, and fails otherwise?- a \== a. no?- a \== b.?- a \== 'a'. no?- a \== X. X = _443

8 Terms with a special notation Sometimes terms look different, but Prolog regards them as identical For example: a and 'a', but there are many other cases Why does Prolog do this? Because it makes programming more pleasant More natural way of coding Prolog programs

9 Arithmetic terms Recall lecture 5 where we introduced arithmetic +, -, <, >, etc are functors and expressions such as 2+3 are actually ordinary complex terms The term 2+3 is identical to the term +(2,3)

10 Arithmetic terms Recall lecture 5 where we introduced arithmetic +, -, <, >, etc are functors and expressions such as 2+3 are actually ordinary complex terms The term 2+3 is identical to the term +(2,3)?- 2+3 == +(2,3).?- -(2,3) == 2-3.?- (4<2) == <(4,2).

11 Summary of comparison predicates = \= == \== =:= =\= Unification predicate Negation of unification predicate Identity predicate Negation of identity predicate Arithmetic equality predicate Negation of arithmetic equality predicate

12 Lists as terms Another example of Prolog working with one internal representation, while showing another to the user Using the constructor, there are many ways of writing the same list?- [a,b,c,d] == [a [b,c,d]].?- [a,b,c,d] == [a,b,c [d]].?- [a,b,c,d] == [a,b,c,d []].?- [a,b,c,d] == [a,b [c,d]].

13 Prolog lists internally Internally, lists are built out of two special terms: [] (which represents the empty list). (a functor of arity 2 used to build non-empty lists) These two terms are also called list constructors A recursive definition shows how they construct lists

14 Definition of prolog list The empty list is the term []. It has length 0. A non-empty list is any term of the form.(term,list), where term is any Prolog term, and list is any Prolog list. If list has length n, then.(term,list) has length n+1.

15 A few examples?-.(a,[]) == [a].?-.(f(d,e),[]) == [f(d,e)].?-.(a,.(b,[])) == [a,b].?-.(a,.(b,.(f(d,e),[]))) == [a,b,f(d,e)].

16 Internal list representation Works similar to the notation: It represents a list in two parts Its first element, the head the rest of the list, the tail The trick is to read these terms as trees Internal nodes are labeled with. All nodes have two daughter nodes Subtree under left daughter is the head Subtree under right daughter is the tail

17 Example of a list as tree Example: [a,[b,c],d]. a... b. d [] c []

18 Examining terms We will now look at built-in predicates that let us examine Prolog terms more closely Predicates that determine the type of terms Predicates that tell us something about the internal structure of terms

19 Type of terms Terms Simple Terms Complex Terms Constants Variables Atoms Numbers

20 Checking the type of a term atom/1 integer/1 float/1 number/1 atomic/1 var/1 nonvar/1 Is the argument an atom? an interger? a floating point number? an integer or float? a constant? an uninstantiated variable? an instantiated variable or another term that is not an uninstantiated variable

21 Type checking: atom/1?- atom(a).?- atom(7). no?- atom(x). no

22 Type checking: atom/1?- X=a, atom(x). X = a?- atom(x), X=a. no

23 Type checking: atomic/1?- atomic(mia).?- atomic(5).?- atomic(loves(vincent,mia)). no

24 Type checking: var/1?- var(mia). no?- var(x).?- X=5, var(x). no

25 Type checking: nonvar/1?- nonvar(x). no?- nonvar(mia).?- nonvar(23).

26 The structure of terms Given a complex term of unknown structure, what kind of information might we want to extract from it? Obviously: The functor The arity The argument Prolog provides built-in predicates to produce this information

27 The functor/3 predicate The functor/3 predicate gives the functor and arity of a complex predicate

28 The functor/3 predicate The functor/3 predicate gives the functor and arity of a complex predicate?- functor(friends(lou,andy),f,a). F = friends A = 2

29 The functor/3 predicate The functor/3 predicate gives the functor and arity of a complex predicate?- functor(friends(lou,andy),f,a). F = friends A = 2?- functor([lou,andy,vicky],f,a). F =. A = 2

30 functor/3 and constants What happens when we use functor/3 with constants?

31 functor/3 and constants What happens when we use functor/3 with constants??- functor(mia,f,a). F = mia A = 0

32 functor/3 and constants What happens when we use functor/3 with constants??- functor(mia,f,a). F = mia A = 0?- functor(14,f,a). F = 14 A = 0

33 functor/3 for constructing terms You can also use functor/3 to construct terms:?- functor(term,friends,2). Term = friends(_,_)

34 Checking for complex terms complexterm(x):- nonvar(x), functor(x,_,a), A > 0.

35 Arguments: arg/3 Prolog also provides us with the predicate arg/3 This predicate tells us about the arguments of complex terms It takes three arguments: A number N A complex term T The Nth argument of T

36 Arguments: arg/3 Prolog also provides us with the predicate arg/3 This predicate tells us about the arguments of complex terms It takes three arguments: A number N A complex term T The Nth argument of T?- arg(2,likes(lou,andy),a). A = andy

37 Strings Strings are represented in Prolog by a list of character codes Prolog offers double quotes for an easy notation for strings?- S = Vicky. S = [86,105,99,107,121]

38 Working with strings There are several standard predicates for working with strings A particular useful one is atom_codes/2?- atom_codes(vicky,s). S = [118,105,99,107,121]

39 Operators As we have seen, in certain cases, Prolog allows us to use operator notations that are more user friendly Recall, for instance, the arithmetic expressions such as 2+2 which internally means +(2,2) Prolog also has a mechanism to add your own operators

40 Properties of operators Infix operators Functors written between their arguments Examples: + - = ==, ;. --> Prefix operators Functors written before their argument Example: - (to represent negative numbers) Postfix operators Functors written after their argument Example: ++ in the C programming language

41 Precedence Every operator has a certain precedence to work out ambiguous expressions For instance, does 2+3*3 mean 2+(3*3), or (2+3)*3? Because the precedence of + is greater than that of *, Prolog chooses + to be the main functor of 2+3*3

42 Associativity Prolog uses associativity to disambiguate operators with the same precedence value Example: Does this mean (2+3)+4 or 2+(3+4)? Left associative Right associative Operators can also be defined as nonassociative, in which case you are forced to use bracketing in ambiguous cases Examples in Prolog: :- -->

43 Defining operators Prolog lets you define your own operators Operator definitions look like this: :- op(precedence, Type, Name). Precedence: number between 0 and 1200 Type: the type of operator

44 Types of operators in Prolog yfx xfy xfx fx fy xf yf left-associative, infix right-associative, infix non-associative, infix non-associative, prefix right-associative, prefix non-associative, postfix left-associative, postfix

45 Operators in SWI Prolog

46 Next lecture Cuts and negation How to control Prolog`s backtracking behaviour with the help of the cut predicate Explain how the cut can be packaged into a more structured form, namely negation as failure